Polysuccinimide and polysuccinimide copolymers are precursors of polyaspartic acid and polyaspartic acid copolymers. Both, as homopolymers and as copolymers, have been formed by a number of methods or processes known in the art. The term "polyaspartic acid" as used herein refers to the free acid and to metal salts of polyaspartic acid regardless of whether the acid or metal salt thereof is a homopolymer, copolymer or a mixture of homopolymers and copolymers. Polyaspartic acid is used in a wide variety of applications such as an antideposition agent, as a plant nutrient uptake facilitator and in personal care products.
Poly-.alpha.,.beta.-D,L-aspartic acid, copolymers of poly-.alpha.,.beta.-D,L-aspartic acid and their salts are generally water soluble, biodegradable polymers useful in a variety of applications such as in water treatment, detergent formulations, oral hygiene products, agriculture, personal care products such as hair and skin care toiletries, and in cosmetics. In many of these applications, highly colored components in the polyaspartic acid or solutions thereof are to be avoided because they impart undesirable color characteristics to the final commercial product.
The processes for the manufacture of polyaspartic acid usually involve two steps. The first step is the polymerization of a monomer such as aspartic acid, monoammonium maleate or maleamic acid to afford polysuccinimide. A copolymer can be formed by polymerization of the above monomers with a co-monomer familiar to those skilled in the art. A mixture of homopolymer and copolymer can be prepared by adjusting the quantities of monomer and co-monomer used in the polymerization reaction or by mixing a homopolymer with a copolymer.
The second step involves the hydrolysis of the polysuccinimide homopolymer or copolymer, or mixtures of homopolymer and copolymer, or metal salts thereof, with aqueous base or saturated steam to afford a polyaspartate homopolymer or copolymer, or mixtures, or metal salts thereof. For base hydrolysis, the polysuccinimide reaction product is typically combined with aqueous ammonium hydroxide, sodium hydroxide, sodium carbonate and the like.
The base hydrolysis of polysuccinimide or copolymers of polysuccinimide to produce poly-.alpha.,.beta.-D,L-aspartic acid and salts of polyaspartic acid has been disclosed. For examples of such hydrolysis see U.S. Pat. Nos. 5,057,597, 5,221,733 and 5,315,010 to Koskan et al., and U.S. Pat. Nos. 5,288,783, 5,286,810 and 5,292,858 to Wood et al.
Polysuccinimide can be produced by the thermal polymerization of aspartic acid as has been described in U.S. Pat. Nos. 5,057,597 and 5,315,010 to Koskan et al.; by the polymerization of ammonium salts of maleic, fumaric and malic acids as described by Koskan et al. in U.S. Pat. Nos. 5,296,578 and 5,219,952, by Wood et al. in U.S. Pat. No. 5,288,783 and by Boehmke in U.S. Pat. No. 4,839,461; and by the microwave induced polymerization of ammonium salts of maleic, fumaric and malic acids as has been described in U.S. Pat. No. 4,696,981 to Harada et al.
The preparation of polysuccinimide by the thermal polymerization of maleamic acid is described in European Patent No. 593,187 (1994) to Groth et al.; by the phosphoric acid catalyzed thermal polymerization of aspartic acid is described by S. W. Fox et al. in Arch. Biochem. Biophys. 86, 274-280 (1960) and in U.S. Pat. No. 5,142,062 to Knebel et al.; by the thermal polymerization of aspartic acid in the presence of carbon dioxide gas is described in U.S. Pat. No. 5,329,020 to Kalota et al.; by the thermal polymerization of aspartic acid in the presence of solvents is described in French Patent No. 2,424,293 to Jacquet et al. and European Patent No. 578,449 (1994) to Paik et al.; and by the thermal polymerization of maleamic acid and ammonium salts of maleic and fumaric acid in the presence of processing aids as described in European Patent No. 593,187 (1994) to Freeman et al.
The hydrolysis of the polysuccinimide produced by each of the above methods results in polyaspartic acid solutions having varying degrees of color. In many instances, the physical properties and performance of the polyaspartic acid polymers themselves or polymer solutions are satisfactory but the products are too highly colored to be considered usable or suitable for use in color sensitive applications.
Copolymers of polysuccinimide can be produced by a variety of methods. The thermal polymerization of aspartic acid with a variety of co-monomers is described in German Patent Application DE 4,221,875 (1994) to Baur et al. Copolymers of polysuccinimide with diamines can be produced by the thermal polymerization of ammonium salts of maleic and fumaric acids with diamines as described in U.S. Pat. No. 5,286,810 to Wood et al.
Copolymers of polysuccinimide with amines can be produced by the reaction of amines with polysuccinimide in an organic solvent as has been described in U.S. Pat. No. 4,363,793 to Jacquet et al., British Patent UK 1,404,814 to Fujimora et al. and by P. Neri et al. in J. Chem. Med. 16, 893-897 (1973); or by the thermal polymerization of ammonium salts of maleic or fumaric acid with amines as described in U.S. Pat. No. 5,292,864 to Wood et al. However, as is the case with the polysucccinimide homopolymers, hydrolysis of the copolymers of polysuccinimide produces highly colored solutions of the copolymers of polyaspartic acid. Consequently, while the physical properties of the copolymers and/or their solutions may be satisfactory, they are not useful in color-sensitive applications.
Methods of preparing low-color solutions of polyaspartic acid have been described. For example, U.S. Pat. No. 5,292,864 to Wood et al. discloses preparing low-color solutions of the sodium salts of polyaspartic acid and copolymers of polyaspartic acid by the treatment of aqueous solutions of polyaspartates with decolorizing agents such as hypochlorite, chlorine, chlorine dioxide, hydrogen peroxide, a peroxycarboxylate or ozone.
However, the methods described by Wood et al. are unsatisfactory on several points. First, the use of chlorinated reagents is environmentally unacceptable. Second, the method described by Wood et al. requires an additional processing step to produce the low-color polyaspartates. The overall process as described by Wood et al., from monomer to final product, is a three step process. Third, in many instances treating the polyaspartate solutions with the decolorizing agents described by Wood et al. changes either the concentration or the pH of the solution, or both. Consequently, additional processing is required to compensate for these changes. These further processing requirements incur additional time and expense.
The art needs a new and improved method of producing low-color polyaspartates and polyaspartate solutions directly from polysuccinimides without the need for additional processing beyond the hydrolysis step already being used. Furthermore, the art needs a new and improved method which does not require the use of environmentally unacceptable chlorinated reagents or expensive ozone equipment.